Electroless copper solutions are commonly used in the electronics industry for depositing metallic copper on substrates such as circuit boards without application of external electrical current. Circuit boards are typically immersed in an electroless copper solution wherein, by means of oxidation-reduction reactions, metallic copper is uniformly deposited on surfaces of the hole walls, thus producing a circuit board having plated through holes (PTH). This provides electrical continuity from one surface of the circuit board to the other. The electroless copper solution is maintained at an elevated pH level under reaction conditions which promote reduction of cupric ions in solution to metallic copper. Formaldehyde typically serves as the reducing agent. The metallic copper plating reaction in an electroless copper solution proceeds as follows: EQU Cu.sup.++ +2HCHO+4OH.sup.- .fwdarw.Cu.sup.0 +H.sub.2 +2HCOO.sup.- +2H.sub.2 O Uniformity of metallic copper deposition is critical to meet circuit board performance standards, since inconsistencies in metallic copper plating may result in defective circuit boards which are prone to failures. The electroless copper solution is therefore frequently or continuously replenished to maintain adequate concentrations of reactants for continuous uniform plating of metallic copper. Since the chemistry of the electroless copper solution is maintained substantially constant, overflow electroless copper solutions contain high levels of dissolved copper and formaldehyde.
Dissolved copper concentrations in discharge solutions are strictly regulated, and electroless copper overflow solutions must be processed to remove substantially all dissolved copper from solution before they can be discharged to the environment. Waste streams comprising dissolved copper are considered hazardous wastes, and they must be handled and disposed of with great care. In many facilities, electroless copper waste is contained and transported to a remote site for disposal. Handling, transport and disposal of electroless copper waste streams as a hazardous waste material involves substantial expense.
Several processes for removal of dissolved copper from solution are known in the art. For example, dithiocarbamates, (DTC) such as sodium dithiocarbamate, are known as metal scavengers and have been used to remove copper and other soluble metals from solution. Most effective metal removal using DTC is achieved when the total metal concentration is in excess of 10 mg/L. DTC is generally available commercially in the form of dimethyl or diethyl derivatives. Insoluble starch xanthate has also been employed in processes for removal of dissolved copper for example, from industrial alkaline rinses and industrial ammonium persulfate copper etchant rinses.
Sodium borohydride (NaBH.sub.4) has been utilized to remove metals from solution by reducing the metal ions to a lower valence state or to the free metal. Metals may redissolve in solution unless the precipitated metals are removed quickly, however, and further wastewater treatment is necessary to remove borate from solution. Sodium hydrosulfite has also been employed as a reducing agent for metal removal from chelated systems. In general, processes for removal of dissolved copper from solutions containing high concentrations of dissolved copper may be prohibitively expensive to operate, require specialized equipment, or involve multiple process stages.
The metallic copper plating reaction may be permitted to proceed so that dissolved copper in electroless copper waste streams is eliminated simply by plating out of solution as metallic copper. This process involves significant inconvenience, however, and it is difficult to control. Electroplating techniques may likewise be implemented to remove dissolved copper from electroless copper baths. Electroplating processes typically require pH adjustment prior to the application of an electrical current. Dissolved copper may be removed from electroless copper rinses by adjusting the pH of solution and passing the dissolved copper rinse solution through an ion exchange column which provides selective removal of dissolved copper. Treatment of electroless copper overflow solutions in ion exchange columns is ineffective, however, due to the significantly higher concentrations of dissolved copper. Accordingly, it is an objective of the present invention to provide a process for removal of dissolved copper from solution which provides substantially complete copper removal and improved process efficiency.
It is another objective of the present invention to provide a process for removal of substantially all dissolved copper from waste streams having dissolved copper concentrations of up to several thousand ppm.
It is still another objective of the present invention to provide a process for precipitation of dissolved copper from electroless copper waste streams as metallic copper which may be partitioned from the purified liquid.
It is yet another objective of the present invention to provide a process for removal of dissolved copper from solution which is easily implemented without requiring substantial equipment, chemical, energy or supervisory requirements.